1. Field of the Invention:
The present invention relates to an electroactivated material suitable for the production of the cathode element of an electrolysis cell, and, more especially, of a cell for the electrolysis of aqueous solutions of alkali metal halides. This invention also relates to such cathode element, per se, and to a process for the production of said materials and cathode elements.
The subject electroactivated materials exhibit a low overvoltage with respect to the hydrogen release reaction at the cathode and consequently permit considerable energy savings.
2. Description of the Prior Art:
In published European Patent Application EP-A-0,132,425 (corresponding to U.S. Pat. No. 4,743,349), a material is described comprising fibers and a binder suitable for producing the cathode element of an electrolysis cell, said material being characterized in that at least a part of the fibers consists of electrically conductive fibers, in that the binder is selected from among the fluoropolymers and in that the resistivity of said material is below 0.4 ohm cm and preferably below 0.1 ohm cm.
The conductive fibers may be carbon fibers and non-conductive fibers, such as asbestos fibers also constitute the described material.
Such prior art material may additionally contain one or more electrocatalytic agents which may be present in the form of powder with a particle size of from 1 to 10 microns. Platinum, palladium, and the nickel-zinc, nickel-aluminum, titanium-nickel, molybdenum-nickel, sulfur-nickel, nickel-phosphorus, cobalt-molybdenum and lanthanum-nickel alloys and couples are among the electrocatalytic agents intended.
In this '425 application, the quantity of electrocatalytic agent could represent up to 50% by weight of the bonded sheet, a content of between 1 and 30% of the said weight being recommended.
However, all of the materials according to this '425 application, although being electroactivated, exhibit at least one of the following disadvantages:
(1) resorting to nonconductive fibers which have been made conductive by metallizing, such as nickel-coated asbestos fibers, or nickel-coated carbon fibers, is not only costly--and this restricts development on an industrial scale--but would be incapable of replacing the electrocatalytic agent when considered strictly from the standpoint of efficiency; furthermore, these fibers can be altered by heat treatments;
(2) resorting to electrochemical deposition, for example of a Ni,Zn alloy, on a precathode sheet (that is to say, a sheet of fibers, composite and deposited onto an elementary cathode, this sheet fulfilling in particular the usual function of a cathode) presents the major disadvantage of activating the sheet only at the surface, with its core being unaffected, and this markedly diminishes the advantages to be expected of a three-dimensional electrode; moreover, since the electroactivator is situated at the diaphragm/precathode interface, most of the hydrogen release takes place at this interface and contributes to the breaking up of the assembly. In addition to this, a sheet of this kind can be coupled with a diaphragm only with difficulty, because of the metal layer formed on its surface;
(3) the incorporation of the Ni-Al compound in the form of powder into materials of this type, in order to produce cathode elements does not permit a satisfactory improvement in their performance, as is shown by the comparison of the voltage values (at the electrolyzer terminals) extrapolated to zero intensity (U.sub.I.fwdarw.o), which are obtained without activation and with activation, respectively, and which are reproduced in the last table of said '425 European application.
This same application also proposes a process for the manufacture of these materials, which consists in preparing a suspension comprising the conductive fibers and the binder, and then removing the liquid medium and drying the sheet obtained. The suspension may also contain non-conductive fibers, pore-forming agents and/or catalytic agents.
The sheet may be formed by filtering the suspension through a highly porous material, under programmed vacuum, and can then be dried for 1 to 24 h at a temperature of between 70.degree. and 120.degree. C., and then bonded by heating to a temperature from 5.degree. to 50.degree. C. above the melting or softening point of the fluoropolymer for a period of time which can range from 2 to 60 min.
Finally, this European application also proposes use of the materials discussed above for the production of composite materials by combining said materials with an elementary cathode consisting of a metal surface. Said combination is carried out by filtering the suspension containing the fibers and the fluoropolymer directly through said elementary cathode, followed by melting of the binder.
Furthermore, European Patent Application No. 86/420184.3 (corresponding to U.S. patent application Ser. No. 892,432, filed Aug. 4, 1986), notes the importance of the monodisperse nature of the lengths of the fibers employed, both from the standpoint of the quality of the microporous materials containing electrically conductive fibers, such as carbon or graphite fibers, as well as from the standpoint of producing such materials on an industrial scale by vacuum filtration of a suspension of fibers and binder.
By "monodisperse distribution" is intended a distribution of lengths such that the length of at least 80%, and advantageously 90%, of the fibers corresponds to the mean length of the fibers to within .+-.20% and, advantageously, to within .+-.10%.
In a preferred embodiment, the mean length of the fibers is, at most, equal to the diameter of the perforations of the perforated rigid substrate onto which the fibrous sheet is deposited.
European Patent Application No. 86/420237.9 (corresponding to U.S. Pat. No. 4,775,551), notes the importance, both from the standpoint of microporosity and from that of the consolidation of the microporous materials, of using certain derivatives based on silica as an agent for forming the network of binder based on a fluoropolymer latex and, more particularly, when carbon or graphite fibers are to be bonded by means of a polytetrafluoroethylene latex.
Nonetheless, need continues to exist in this art for yet further improved electrode material.